Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session L2: Focus Session: Symmetry Breaking & Dissociation in Small Molecules |
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Chair: David Schultz, Oak Ridge National Laboratory Room: Imperial Center |
Thursday, May 27, 2010 2:00PM - 2:30PM |
L2.00001: Symmetry Breaking in Dissociative Electron Attachment to Molecules Invited Speaker: Modern experimental techniques allow detailed measurements of the dynamics of reactions, recording in coincidence, both electron angular distributions and the momenta of all charged molecular fragments. These studies are an excellent probe of the dynamics in the interaction of photons and electrons with molecules and the subsequent flow of energy between the vibrational modes of the system. These measurements pose a new challenge to theoretical predictions. Much of the theoretical work in dissociative electron attachment to molecules has concentrated on diatomic systems. This can lead to ``rules'' that are not followed when moving to polyatomic systems. In this talk, I will discuss cases of symmetry breaking in dissociation of molecules, where these diatomic ``rules'' are violated. I will illustrate this in the case of dissociative attachment in formic acid, HCCH and HCN/HNC, where the electron attaches to a $\pi^*$ antibonding orbital, but the fragments are produced in symmetric final states. [Preview Abstract] |
Thursday, May 27, 2010 2:30PM - 3:00PM |
L2.00002: Symmetry breaking puzzles in inner-shell photoionization of symmetric linear molecules Invited Speaker: Core-level photoionization of small molecules, followed by Auger decay, generally leads to molecular fragmentation. Modern momentum imaging (COLTRIMS) techniques, combined with coincident measurement of all charged particles produced by a single photoabsorption event, allow one to reconstruct photo- and Auger electron angular distributions in the body frame, which are generally far richer than conventionally measured laboratory-frame angular distributions. In this talk, I will discuss two cases where such measurements on symmetric linear target molecules have revealed interesting symmetry paradoxes. The first case involves K-shell ionization of molecular nitrogen where experimental measurements raise familiar questions about whether the photoprocess creates a localized hole at one of two equivalent, but spatially well-separated sites or rather a delocalized hole, smeared over the equivalent sites, preserving the symmetry of the molecule. The second example involves carbon 1s ejection from the symmetric CO$_2$ molecule and the origin of observed asymmetry in the molecular-frame photoelectron angular distributions. In both cases, theoretical calculations assist in interpreting the experimental findings. [Preview Abstract] |
Thursday, May 27, 2010 3:00PM - 3:12PM |
L2.00003: Asymmetry in the momentum distribution of H+$\mathrm{p}$ from dissociative ionization of $\mathrm{H_{2}}$ controlled by the carrier-envelope phase of a few-cycle pulse Shuo Zeng, Fatima Anis, B.D. Esry We present theoretical results on the interaction of $\mathrm{H_{2}}$ with an ultrashort linearly polarized laser pulse of 6 fs duration. We investigate the carrier-envelope phase effects of this ultrashort laser pulse on the asymmetry of the momentum distribution of H+$\mathrm{p}$ along the laser polarization direction. To do so, we model the $\mathrm{H_{2}}$ ionization by launching a coherent wavepacket on $\mathrm{H_{2}^{+}}$ potential curves at each field maximum, and then propagate wavepackets in time on $\mathrm{H_{2}^{+}}$ Born-Oppenheimer potential curves coupled by the laser. Nuclear rotation and vibration are both included in the Schr{\"o}dinger equation for $\mathrm{H_{2}^{+}}$ . Our results will be compared to a recent experimental measurement [1]. We will also compare our results to calculations neglecting rotation and discuss the limitations of such a model.\\[4pt] [1] Manuel Kremer \textit{et al.}, Phys. Rev. Lett. 103.213003 (2009) [Preview Abstract] |
Thursday, May 27, 2010 3:12PM - 3:24PM |
L2.00004: Controlling the dissociation of an HD$^{+}$ beam with intense two-color laser field I. Ben-Itzhak, J. McKenna, F. Anis, D. Ray, B. Gaire, M. Zohrabi, D. Ursrey, C.L. Cocke, K.D. Carnes, B.D. Esry Electron localization on a specific nucleus during strong-field dissociation of a molecular-ion is controlled by the relative phase between the 790 and 395 nm components of a linearly-polarized ultrashort laser pulse. We have observed both spatial and channel asymmetries experimentally for an HD$^{+}$ target. The spatial asymmetry, which has been observed before, has been understood as being due to the breaking of the spatial symmetry of the driving field. The channel asymmetry, namely the controlled dissociation into either H$^{+}$+D(1s) or H(1s)+D$^{+}$, is independent of the spatial asymmetry and is not as easily understood in the language of driving field asymmetry. We will discuss these first measurements of this effect as well as an attempt to understand both the spatial and channel asymmetries within a single, unified picture. [Preview Abstract] |
Thursday, May 27, 2010 3:24PM - 3:36PM |
L2.00005: Theoretical study of photodetachment of HOCO$^-$ near threshold Shungo Miyabe, C. William McCurdy, Thomas N. Rescigno Continetti and Lu (Phys.~Rev.~Lett. 99, 113005 (2007)) have shown that the photoelectron kinetic energy distribution from HOCO$^-$ exhibits two sharp peaks near threshold accompanied by a broader feature raging from 0.4 to 1.9 eV. They have argued that the threshold peaks are s- and p- wave shape resonances. In contrast, here we show that these resonances can be attributed to the vibrational levels of a dipole-bound state of trans-HOCO$^-$. Our MP2 calculations give a value of 2.50 Debye for the dipole moment of trans-HOCO$^-$. The electron binding energy for the dipole-bound anion was found to be 0.0012 eV at the configuration-interaction level of theory with single and double excitations. We have also calculated the fixed-nuclei photodetachment amplitudes using variationally obtained electron-molecule scattering wavefunctions, which identifies the broad feature as an A$''$-shape resonance. [Preview Abstract] |
Thursday, May 27, 2010 3:36PM - 3:48PM |
L2.00006: Imaging intense-field dissociation of HeH+ as an analog for ionization D. Ursrey, B.D. Esry Although intense field ionization is generally regarded as well understood, there are still questions about how this understanding meshes with the dynamics predicted by the time-dependent Schr\"odinger equation (TDSE). We would like to answer these questions. In particular, we would like to study how the photoelectron spectrum develops over time. Is an ionized electron produced with zero kinetic energy as predicted by tunneling models? Can this question be answered in an experimentally verifiable way? As a first step, we use $\mathrm{HeH^{+}}$ as an analog to an atomic system and image the dissociating nuclear wavepacket, which allows us to use femtosecond rather than attosecond pulses. We use $\mathrm{HeH^{+}}$ because its nuclear TDSE is essentially identical to that of the electron in a one-electron atom, except that there is no Coulomb potential and the effective mass is much larger. We will study the time dependence of the above threshold dissociation of $\mathrm{HeH^{+}}$ in an intense field and discuss possible interpretations of our results within the context of intense field ionization theories. [Preview Abstract] |
Thursday, May 27, 2010 3:48PM - 4:00PM |
L2.00007: Clear evidence of zero-photon dissociation of H$_{2}^{+}$ in intense few cycle laser pulses B. Gaire, J. McKenna, A.M. Sayler, F. Anis, M. Zohrabi, Nora G. Johnson, J.J. Hua, K.D. Carnes, B.D. Esry, I. Ben-Itzhak Evidence of net zero-photon dissociation is elusive, since the low kinetic energy release, which is the signature of this process, can also be due to resonance enhanced multi-photon ionization (REMPI) of H$_{2}$ [Posthumus \textit{et al}., Phys. Rev. Lett. \textbf{101}, 233004 (2008)]. To eliminate the competing REMPI process we have studied an H$_{2}^{+}$ beam target. Explicitly, we measured very low kinetic energy release (down to 0 eV) upon dissociation of H$_{2}^{+}$ using a coincidence three dimensional momentum imaging technique. Our experimental findings are supported by the solutions of the time-dependent Schr\"{o}dinger equation for the conditions used in the experiment. Our results suggest that the zero-photon dissociation mechanism, most visible at $\sim $10$^{13}$ W/cm$^{2}$, is enhanced for short laser pulses ($\le $10 fs). [Preview Abstract] |
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